Skip to main content

Questions tagged [general-relativity]

Questions about relativistic theory of gravity. General relativity describes gravity as a geometrical property of space and time.

39 questions with no upvoted or accepted answers
Filter by
Sorted by
Tagged with
5 votes
0 answers
68 views

Is gravitational lensing "additive" in a line of sight?

As usually depicted, gravitational lensing implies, at least, three objects: a distant source, an observer, and a "massive enough" object in the middle, where enough is the mass needed to ...
nuwe's user avatar
  • 771
4 votes
0 answers
69 views

What does it exactly mean for spacetime to have no global symmetries?

Are there spacetimes or metrics with no global symmetries? Spacetimes/metrics with no global Poincaré, Lorentz, diffeomorphism, CPT, translational and gauge invariances? And if there are, what does it ...
vengaq's user avatar
  • 1,017
4 votes
0 answers
87 views

Why does the outside edge of a Kerr black hole ergosphere rotate at the same speed as the inner edge?

Reading through the Wikipedia on a Kerr black hole ergosphere, and this nice question on the maximal spin rate of a black hole leads me to ask: why does the outer edge of the ergosphere rotate at the ...
joseph.hainline's user avatar
4 votes
0 answers
101 views

On the geodesics of the metric $ds^2=-\rho^2d\alpha^2+d\rho^2$ and the constant $l=\rho^2\frac{d\alpha}{d\tau}$

In my assignment I have to deal with the 2D spacetime metric $$ds^2=-\rho^2d\alpha^2+d\rho^2.$$ During this assignment we have shown that the constant $$l=\rho^2\frac{d\alpha}{d\tau}$$ is a constant ...
Soof_fie's user avatar
  • 183
4 votes
1 answer
149 views

About the strange case of Bonnor beams

Bonnor beams are light beams of infinite extent and small thickness. They have an associated spacetime (related to their energy and momentum which shapes the stress-energy tensor) of which Bonnor ...
Deschele Schilder's user avatar
3 votes
0 answers
68 views

Gravitational recoil with stars/planets...?

When two black holes are merging, the resulting merge can be ejected if one of the black holes had less mass than the other one, so the gravitational waves emitted by both of them is unbalanced, and ...
vengaq's user avatar
  • 1,017
3 votes
0 answers
95 views

Kinetic energy in cosmology?

Spacetime expands at an accelerated rate due to the Hubble flow. In many papers that I've read, objects coupled to the Hubble flow are treated as if they have some velocity and kinetic energy ...
vengaq's user avatar
  • 1,017
3 votes
0 answers
79 views

Deviations of conservation laws in the context of cosmological evolution?

If energy is "not conserved" in General Relativity (or at least, it is difficult to define it) in the context of an accelerating expanding spacetime (like it happens in our Universe), are ...
vengaq's user avatar
  • 1,017
3 votes
0 answers
92 views

Does the presence of dark matter in the Milky Way significantly distort our parallax based measurements of distance?

According to accepted astronomical wisdom, dark matter represents a large fraction of the total mass of the Milky Way. And thanks to Papa Einstein, we know that light rays can be bent when passing ...
MarkVonTexas's user avatar
3 votes
0 answers
65 views

Does General Relativity predict anything about the creating of a singularity, once enough matter collapses inward to form an event horizon?

Ignoring quantum gravity, which we don't know much about, current theory (GR) states that an event horizon forms when there is enough matter/energy density to create it, and logically, a singularity I ...
Stilez's user avatar
  • 1,022
3 votes
0 answers
102 views

Is TDB monotonic? Does it or any of its derivatives have yearly discontinuities with respect to an atomic clock?

This in-depth answer mentions that ...TDB is updated every year. As technology improves, these subtle updates change the timing of past events. This makes me wonder... Does this updating result ...
uhoh's user avatar
  • 30.7k
2 votes
0 answers
44 views

Which static spherically symmetric perfect fluid solutions of Einstein field equations are known to be suitable for stellar models?

Wikipedia's (Static spherically symmetric perfect fluid) short list ends in 2005 and lists only in total 5 such solutions. Does somebody know more?
JanG's user avatar
  • 129
2 votes
0 answers
85 views

Parker Solar Probe and clock correction

Due to the dynamic changes in speed and the value of the gravitational field in which the Parker Solar Probe stays, the clocks should have the errors due to relativistic effects. https://physics....
user1785960's user avatar
2 votes
0 answers
120 views

Order of magnitude of this phase space invariant?

So I wanted know the order of magnitude of this phase space invariant (equation $3.31a$ )$R$ in our universe (FLRW metric): $$R = \int_{p} \frac{\mathcal{N}(x,p,t)}{\mathcal{E}} dp_x dp_y dp_z$$ ...
More Anonymous's user avatar
2 votes
0 answers
101 views

How String Theory calculates the Entropy of the Black Hole in a Background dependent way?

How does string theory get an agreement to the Hawking-Bekenstein equations to calculate the quantum entropy of a black hole in a background dependent way? Is there any sort of area parameter in ...
Loop Quantum Gravity's user avatar
2 votes
0 answers
83 views

If time is not absolute and depends on the gravitational field, then why we keep using our own (particular) time in cosmology?

Sorry if my question is not really clear, but indeed it is not clear in my mind neither. But maybe some of you can help. Time is not absolute and can be affected by the gravitational potential. Now ...
user avatar
2 votes
0 answers
48 views

How to compare the observation with the theoretically predicted result?

On the Wikipedia Article on “Geodesics in general relativity”, it says the following: “Thus, for example, the path of a planet orbiting a star is the projection of a geodesic of the curved 4-D ...
SOQEH's user avatar
  • 21
1 vote
0 answers
32 views

What is called following two metric's signature if they are not Lorentz signatures?

I make two different substitutions (m(r)=m1(r), m2(r)) to the modified Schwarzschild metrics: $${\it m1} \left( r \right) =1/2\,r{c}^{2} \left( 1+\sqrt {{\frac {r{c}^ {2}-2\,MG}{r{c}^{2}}}} \right) {G}...
Hannu Poropudas's user avatar
1 vote
0 answers
68 views

What does the upper and lower indices in metric tensor means?

When we have metric g that can be written as $$\begin{vmatrix}g_1 & g_2 & g_3\\ g_4 & g_5 & g_6 \\ g_7 & g_8 & g_9\end{vmatrix}$$ What does the $g^{\alpha\beta}$ and $g_{\alpha\...
Graphenjoyer's user avatar
1 vote
0 answers
86 views

If astronomers didn't know about redshift could we still determine the universe is expanding?

What other evidence is there for the expansion of the universe? Would the equations of physics look wrong without it?
user avatar
1 vote
0 answers
36 views

Modelling dark energy as a potential?

Dark energy is the cosmological constant.However can we model dark energy as some form of field with a corresponding potential?
Volpina's user avatar
  • 111
1 vote
0 answers
55 views

Does the (relative) distance of the photon sphere from the center of a black hole vary according its mass?

I know that the photon circle or photon sphere is located at around 1.5 times the radius of the Schwarzschild event horizon, but wouldn't it be comparatively further out for smaller black holes? After ...
Kurt Hikes's user avatar
  • 5,177
1 vote
0 answers
68 views

Non-homogeneous and anisotropic metric and laws of physics...?

In this popular science article, they say that if our universe resulted to be non-uniform (that is highly anisotropic and inhomogeneous) then the fundamental laws of physics could change from place to ...
vengaq's user avatar
  • 1,017
1 vote
0 answers
27 views

Spacetimes where symmetries vary from place to place?

Are there spacetimes or metrics where symmetries (like Poincaré, Lorentz, diffeomorphism, translational... invariances) are only local and the symmetries of one local neighbourhood are not, a priori, ...
vengaq's user avatar
  • 1,017
1 vote
0 answers
130 views

In between Classical Gravitational Deflection (2GM/rv^2) and General Relativity (4GM/rc^2)

While researching online I've found two expressions for the angle of deflection by gravitational lensing: $\dfrac{2GM}{rv^2},$ Classical(Newtonian) prediction $\dfrac{4GM}{rc^2},$ Relativistic ...
Inigo Montoya's user avatar
1 vote
0 answers
35 views

Does the Schwarzschild metric solution require two (A and B, or g11 and g44) or four components (g11, g22, g33 and g44)?

Different places on the web imply that Schwarzschild's metric uses four components or separate equations, similar to how Einstein's full set requires 6 or 10, or they say only two are needed. Is it 2 ...
Kurt Hikes's user avatar
  • 5,177
1 vote
0 answers
50 views

Why does Titan's (relatively) rapid migration away from Saturn cause the planet to wobble faster, and eventually (maybe) tip onto its side?

Over the past year and a half, multiple articles in the popular press have come out talking about how unexpectedly rapidly Titan is moving away from Saturn, and how this is causing Saturn to wobble ...
Kurt Hikes's user avatar
  • 5,177
1 vote
0 answers
102 views

What happens inside a massive collapsing shell?

I have already asked this on the physics site. I am not sure where it fits better. Consider a massive spherical shell collapsing under its own gravity. Say the mass is that of one galaxy. The ...
user avatar
1 vote
0 answers
60 views

How much does the Higgs field contribute to the energy of empty space?

The Higgs field was not spontaneously broken when the universe was young and hot (like I am not...:)). At the very high temperature that existed back then, the Higgs' energy of the Higgs field was non-...
Deschele Schilder's user avatar
1 vote
0 answers
70 views

Questions on dark matter observations in galaxy clusters

For cluster of galaxies we can observe velocity distributions and so we can deduce mass from multiwavelength observations. I suppose total mass of the cluster could be deduced by observing velocities ...
AlbertBranson's user avatar
1 vote
0 answers
67 views

is it possible to prove some version of the "cosmic censorship hypothesis" of Roger Penrose which proposes that this is impossible?

Can singularities not hidden behind an event horizon, known as "naked singularities", arise from realistic initial conditions, or is it possible to prove some version of the "cosmic censorship ...
Darth Ewok's user avatar
0 votes
0 answers
22 views

Is the hypothetical '(Don) Page curve' of evaporating black holes equal in time, so to speak, on both sides of the curve?

I have heard that Hawking radiation is slow at first, but rapidly increases when the black hole becomes much smaller... (I forget the formula...) Does that mean that the second half of Page's curve, ...
Kurt Hikes's user avatar
  • 5,177
0 votes
0 answers
33 views

Is the difference of redshift parameter z able to prove expansion is accelerating in flat FLRW universe model?

Sorry for my bad english, this is not my mother tongue. Recently, I've encountered a problem in cosmology class that is about FLRW universe. Assuming that the universe we inhabit is composed of non-...
ALLin's user avatar
  • 1
0 votes
0 answers
60 views

$v_{GW}$ near a binary star?

In empty space, at large distances from masses and at small amplitudes, GWs propagate at the speed of light. This does not apply near the source, because there is a large mass and the amplitude of the ...
9herbert9's user avatar
  • 101
0 votes
0 answers
139 views

Does the other side of the Big Bang factor into JWST observations?

Our (roughly) 13.6 billion light year view to the point of origin (big bang) is just along a radial axis. Assuming most matter ejected in a (roughly) spherical pattern, the diameter of the universe is ...
TonyG's user avatar
  • 109
0 votes
0 answers
31 views

Does a gravitational lens around a body look 3-dimensional?

Suppose we approach a neutron star or a stellar black hole close enough, is the gravitational lens around it 3-dimensional (like a tunnel) or is it 2-dimensional? It's 3-dimensional as if you entered ...
Greenhorn's user avatar
  • 293
0 votes
0 answers
59 views

How would you define "space" in GR (General Relativity) and in QFT (Quantum Field Theory)?

Is there a univocal definition of space that can be accepted in GR and in QFT? In GR space is actually "Spacetime", a 4-dimensional entity that includes time. In this case it's impossible to ...
NikoEraser's user avatar
0 votes
0 answers
79 views

Does time flow in a Minkowski spacetime?

In a spacetime where the stress-energy tensor is null (no energy, no matter, no entropy), I wonder whether any motion of the variable time in the phase-space is well defined. The arrow of time, meant ...
user avatar
0 votes
1 answer
97 views

Why does the Hubble flow does not carry photons with it?

I have this naive question. We know that the expansion of the universe "takes" or "carries" the galaxies with it, separating them, but this doesn't apply to photons. My guess is ...
xkudsraw's user avatar